1. Field of the Invention
This invention relates to an imaging optical system used in the optical head of an optical memory device such as a video and audio disk, an optical disk or an optical card, and in particular to a both-surface aspherical single lens which has an imaging magnification of -1/4 to -1/6 whose NA is of the order of 0.45, and in which correction of abberations has been made over a relatively wide field of view. Particularly, in the both-surface aspherical single lens according to the present invention, correction of aberrations is made over a wide field of view and therefore, an imaging optical system can be constituted by a single lens and thus, this invention relates to a lens element suitable for the compact optical system of an optical head forming a finite imaging system.
2. Related Background Art
A finite imaging optical system in which a collimator lens and an objective are made integral for the purpose of reducing the cost of the imaging optical system for an optical memory and which is used at an imaging magnification -1/4.33 is disclosed in Japanese Laid-Open Patent Application No. 26917/1985. The optical system disclosed therein is a lens system comprising four groups of four lenses, and has an advantage that the number of lens barrels can be one for a lens construction usually comprised of a group of two collimator lenses and three groups of three objectives. However, in this lens construction, as compared with the usual lens construction, the number of lenses has only been reduced by one and a sufficient reduction in cost could not be achieved.
On the other hand, single lenses having both surfaces made aspherical as objectives for optical memories are disclosed in U.S. Pat. No. 4,027,952, U.S. Pat. No. 4,449,792, Japanese Laid-Open Patent Application No. 201210/1982, Japanese Laid-Open Patent Application No. 68711/1983, U.S. Pat. No. 4,571,034 and Japanese Laid-Open Patent Application No. 120310/1985. These both-surface aspherical single lenses only have an imaging magnification of -1/17.7 at greatest, and when the lenses of this type are used at an imaging magnification of -1/4 to -1/6 and at a numerical aperture (NA) of the order of 0.45, the imaging performance thereof deteriorates remarkably. Further, U.S. Pat. No. 4,449,792, Japanese Laid-Open Patent Application No. 201210/1982, Japanese Laid-Open Patent Application No. 68711/1983, U.S. Pat. No. 4,571,034 and Japanese Laid-Open Patent Application No. 120310/1985 pay attention chiefly to spherical aberration and coma and are designed to make a near-axis aberration correction of the order of 0.1-0.2 mm.phi. on the disk surface. That is, any of these are designed to be used with a collimator lens and thus, these documents relate to a system whereby the objectives are moved in parallel as a unit in a parallel light beam during the tracking, and the abberation of only a minute area near the axis need be corrected. However, in a finite imaging system having an imaging magnification of the order of -1/4 to -1/6, the parallel movement of the lenses during the tracking results in great off-axis imaging for the lenses and therefore, as compared with the conventional infinity imaging system, it is necessary to design the range of aberration correction of the lens itself widely. That is, an imaging performance approximate to the diffraction limit becomes necessary over the order of 0.4 to 0.5 mm.phi. on the disk surface, and correction of astigmatism becomes requisite in addition to correction of spherical aberration and coma. In the aforementioned U.S. Pat. No. 4,027,952, description is made of the correcting conditions under which astigmatism and curvature of the image field become equal to zero, and two examples of a pickup lens of a meniscus shape used at an imaging magnification of -1/17.7 to -1/20.2 are disclosed as embodiments. However, these lenses have their focal length designed to be of the order of 7.5 mm, and have a disadvantage that if the focal length is of the orders of 4-5 mm to make the lenses compact, a sufficient working distance cannot be obtained because the second surface in particular is concave relative to the disk surface.